This invention generally relates to the field of digitally controlled printing devices, and in particular to liquid ink printheads in which a liquid drop is selected for printing by the asymmetrical application of heat to a jet of fluid.
Ink jet printing has become recognized as a prominent contender in the digitally controlled, electronic printing arena because, e.g., of its non-impact, low noise characteristics and system simplicity. For these reasons, ink jet printers have achieved commercial success for home and office use and other areas.
Ink jet printing mechanisms can be categorized as either continuous (CIJ) or Drop-on-Demand (DOD). U.S. Pat. No. 3,946,398, which issued to Kyser et al. in 1970, discloses a DOD ink jet printer which applies a high voltage to a piezoelectric crystal, causing the crystal to bend, applying pressure on an ink reservoir and jetting drops on demand. Piezoelectric DOD printers have achieved commercial success at image resolutions greater than 720 dpi for home and office printers. However, piezoelectric printing mechanisms usually require compiles high voltage drive circuitry and bulky piezoelectric crystal arrays, which are advantageous in regard to number of nozzles per unit length of printhead, as well as the length of the printhead. Typically, piezoelectric printheads contain at most a few hundred nozzles.
Great Britain Patent No. 2,007,162, which issued to Endo et al. in 1979, discloses an electrothermal drop-on-demand ink jet printer that applies a power pulse to a heater which is in thermal contact with water based ink in a nozzle. A small quantity of ink rapidly evaporates, forming a bubble, which causes a drop of ink to be ejected from small apertures along an edge of a heater substrate. This technology is known as thermal ink jet or bubble jet.
Thermal ink jet printing typically requires that the heater generates an energy impulse enough to heat the ink to a temperature near 400xc2x0 C. which causes a rapid formation of a bubble. The high temperatures needed with this device necessitate the use of special inks, complicates driver electronics, and precipitates deterioration of heater elements through cavitation and kogation. Kogation is the accumulation of ink combustion by-products that encrust the heater with debris. Such encrusted debris interferes with the thermal efficiency of the heater and thus shorten the operational life of the printhead. And, the high active power consumption of each heater prevents the manufacture of low cost, high speed and page wide printheads.
Continuous ink jet printing itself dates back to at least 1929. See U.S. Pat. No. 1,941,001 which issued to Hansell that year.
U.S. Pat. No. 3,373,437 which issued to Sweet et al. in March 1968, discloses an array of continuous ink jet nozzles wherein ink drops to be printed are selectively charged and deflected towards the recording medium. This technique is known as binary deflection continuous ink jet printing, and is used by several manufacturers, including Elmjet and Scitex.
U.S. Pat. No. 3,416,153, issued to Hertz et al. in December 1968. This patent discloses a method of achieving variable optical density of printed spots, in continuous ink jet printing. The electrostatic dispersion of a charged drop stream serves to modulate the number of droplets which pass-through a small aperture. This technique is used in ink jet printers manufactured by Iris.
U.S. Pat. No. 4,346,387, entitled METHOD AND APPARATUS FOR CONTROLLING THE ELECTRIC CHARGE ON DROPLETS AND INK JET RECORDER INCORPORATING THE SAME issued in the name of Carl H. Hertz on Aug. 24, 1982. This patent discloses a CIJ system for controlling the electrostatic charge on droplets. The droplets are formed by breaking up of a pressurized liquid stream, at a drop formation point located within an electrostatic charging tunnel, having an electrical field. Drop formation is effected at a point in the electrical field corresponding to whatever predetermined charge is desired. In addition to charging tunnels, deflection plates are used to actually deflect the drops. The Hertz system requires that the droplets produced by charged and then deflected into a gutter or onto the printing medium. The charging and deflection mechanisms are bulky and severely limit the number of nozzles per printhead.
Until recently, conventional continuous ink jet techniques all utilized, in one form or another, electrostatic charging tunnels that were placed close to the point where the drops are formed in the stream. In the tunnels, individual drops may be charged selectively. The selected drops are charged and deflected downstream by the presence of deflector plates that have a large potential difference between them. A gutter (sometimes referred to as a xe2x80x9ccatcherxe2x80x9d) is normally used to intercept the charged drops and establish a non-print mode, while the uncharged drops are free to strike the recording medium in a print mode as the ink stream is thereby deflected, between the xe2x80x9cnon-printxe2x80x9d mode and the xe2x80x9cprintxe2x80x9d mode.
Recently, a novel continuous ink jet printer system has been developed which renders the above-described electrostatic charging tunnels unnecessary. Additionally, it serves to better couple the functions of (1) droplet formation and (2) droplet deflection. That system is disclosed in the commonly assigned U.S. Pat. No. 6,079,821 entitled CONTINUOUS INK JET PRINTER WITH ASYMMETRIC HEATING DROP DEFLECTION filed in the names of James Chwalek, Dave Jeanmaire and Constantine Anagnostopoulos, the contents of which are incorporated herein by reference. This patent discloses an apparatus for controlling ink in a continuous ink jet printer. The apparatus comprises an ink delivery channel, a source of pressurized ink in communication with the ink delivery channel, and a nozzle having a bore which opens into the ink delivery channel, from which a continuous stream of ink flows. Periodic application of weak heat pulses to the stream by a heater causes the ink stream to break up into a plurality of droplets synchronously with the applied heat pulses and at a position spaced from the nozzle. The droplets are deflected by increased heat pulses from the heater (in the nozzle bore) which heater has a selectively actuated section, i.e. the section associated with only a portion of the nozzle bore. Selective actuation of a particular heater section, constitutes what has been termed an asymmetrical application of heat to the stream. Alternating the sections can, in turn, alternate the direction in which the asymmetrical heat is supplied and serves to thereby deflect ink drops, inter alia, between a xe2x80x9cprintxe2x80x9d direction (onto a recording medium) and a xe2x80x9cnon-printxe2x80x9d direction (back into a xe2x80x9ccatcherxe2x80x9d). The patent of Chwalek et al. thus provides a liquid printing system that affords significant improvements toward overcoming the prior art problems associated with the number of nozzles per printhead, printhead length, power usage and characteristics of useful inks.
Asymmetrically applied heat results in stream deflection, the magnitude of which depends on several factors, e.g. the geometric and thermal properties of the nozzles, the quantity of applied heat, the pressure applied to, and the physical, chemical and thermal properties of the ink. Although solvent-based (particularly alcohol-based) inks have quite a good deflection patterns, and achieve high image quantity in asymmetrically heated continuous ink jet printers, water-based inks are more problematic as disclosed in commonly assigned U.S. application Ser. No. 09/451,790 filed Dec. 1, 1999 in the names of Trauernicht et al. The water-based inks do not deflect as much, thus their operation is not robust. In order to improve the magnitude of the ink droplet deflection within continuous ink jet asymmetrically heated printing systems there is disclosed in commonly assigned U.S. application Ser. No. 09/470,638 filed Dec. 22, 1999 in the names of Delametter et al. a continuous ink jet printer having improved ink drop deflection, particularly for aqueous based inks, by providing enhanced lateral flow characteristics, by geometric obstruction within the ink delivery channel.
The invention to be described herein builds upon the work of Chwalek et al., and in accordance with certain embodiments of the invention is an alternate, simpler, design to that of Delametter et al. for constructing continuous ink jet printheads in a variety of materials that are low-cost to manufacture and preferably for printheads that can be made page wide. Alternatively, in accordance with other embodiments of the invention which make use of the improvements disclosed by Delametter et al. improved performance can be achieved.
Although the invention may be used with ink jet printheads that are not considered to be page wide printheads there remains a widely recognized need for improved ink jet printing systems, providing advantages for example, as to cost, size, speed, quality, reliability, small nozzle orifice size, small droplets size, low power usage, simplicity of construction in operation, durability and manufacturability. In this regard, there is a particular long-standing need for the capability to manufacture page wide, high-resolution ink jet printheads. As used herein, term xe2x80x9cpage-widexe2x80x9d refers to printheads of a minimum length of about four inches. High-resolution implies nozzle density, for each ink color, of a minimum of about 300 nozzles per inch to a maximum of about 2400 nozzles per inch.
To take full advantages of page wide printheads with regard to increased printing speed, they must contain a large number of nozzles. For example, a conventional scanning type printhead may have only a few hundred nozzles per ink color. A four inch page wide printhead, suitable for the printing of photographs, should have a few thousand nozzles. While a scanned printhead is slowed down by the need for mechanically moving it across the page, a page wide printhead is stationary and paper moves past it. The image can theoretically be printed in a single pass, thereby substantially increasing the printing speed.
It is therefore an object of the invention to provide an improved CIJ printhead and method of printing using same.
In accordance with a first aspect of the invention, there is provided a continuous ink jet printhead comprising a substrate including an ink delivery channel having ink under pressure in a relief portion formed in the substrate; a thin membrane that comprises an overhang from the relief portion of the substrate, the thin membrane being substantially thinner than a thickness of the substrate and the overhang extending from the relief portion with a dimension OH; a nozzle bore which opens into the ink delivery channel to establish a continuous flow of ink in a stream from the nozzle bore, the nozzle bore being formed in the thin membrane at the overhand and having an exit opening with a respective diameter dimension, D; a heater adjacent the nozzle bore, the heater adapted to produce asymmetric heating of the stream of ink to control direction of the stream between a print direction and a non-print direction; and the nozzle bore being characterized by a dimensional relationship wherein the overhang dimension OH is related to the diameter dimension of the exit opening so that OH greater than =xc2xd D; and wherein thickness t, of the membrane within which the nozzle bore is formed is related to the diameter dimension of the exit opening so that t less than =0.33D.
In accordance with a second aspect of the invention, there is provided a continuous ink jet printhead comprising a nozzle bore formed in a thin membrane that overhangs from a relief portion of a substrate, the thin membrane being of thickness t to define the thickness of the nozzle bore and the nozzle bore being spaced from the relief portion of the substrate with a dimension OH, the nozzle bore having a respective diameter dimension D and characterized in that OH greater than =xc2xd D; and wherein t less than =0.33D.
In accordance with a third aspect of the invention, there is provided a method of operating a continuous ink jet printhead comprising providing a substrate having plural ink delivery channels formed therein each channel terminating at a respective nozzle bore, each nozzle bore being formed in a thin membrane that comprises an overhang from a relief portion of the substrate, the thin membrane being substantially thinner than the thickness of the substrate and the overhang extending from the relief portion with a dimension OH, the nozzle bore having a respective diameter dimension D, and the thin membrane having a thickness t, and wherein the overhand dimension is related to the diameter dimension so that OH greater than =xc2xd D and wherein t less than =0.33D; moving ink under pressure from the ink delivery channels formed in the substrate to each of the nozzle bores to cause ink to flow continuously from the nozzle bores; and selectively effecting collection of certain ink droplets in collection devices associated with the nozzle bores so that ink droplets not collected by the collection devices form a predetermined image on a receiver sheet.
These and other objects, features and advantages of the present invention will become apparent to those skilled in the art upon reading of the following detailed description when taken in conjunction with the drawings wherein there are shown and described illustrative embodiments of the invention.